The present invention relates generally to color television signal generating apparatuses and more particularly to a color television signal generating apparatus comprising a camera tube, a color-resolving striped filter provided on the camera tube, and a circuit for processing a signal obtained from the camera tube.
Among the simple types of known color television cameras, there is a so-called single-tube type in which a single pickup or camera tube, having a color-resolving striped filter in its optical system, is used to generate luminance signals and color signals. Also, a color television camera has two tubes in one pickup or camera tube. On tube is used for generating luminance signals, and the other tube has a color-resolving striped filter within its optical system to generate color signals.
In either type of the above mentioned color television cameras, the color-resolving striped filters are of the phase-separation or the frequency-separation system.
In a color-resolving striped filter of the phase-separation type, however, there has been the disadvantageous requirement that the color-resolving striped filter have a complicated organization, including index stripes. Another disadvantageous requirement is that a complicated has been required for generating sampling pulses on the basis of information obtained from these index stripes. A further problem is that noise results in the conversion of color information signals by a "sampling hold" of a dot-sequential system. In this system, a signal is obtained by sampling and inadvertently introducing noise of high frequency into simultaneous color information signals included in the dot-sequential. The color information signals become stretched along the time axis and are converted into noise of conspicuously low frequency, whereby the signal-to-noise ratio becomes low.
A color-resolving striped filter of the frequency-separation system does not encounter the above described difficulties accompanying a known color-resolving striped filter of the phase-separation system. However, there are interference fringes (moire), due to various causes, since two sheets of striped filters of different space frequency values are fabricated in combination. In addition, the frequency fluctuation of a carrier wave generated in the output signal, as a result of non-linearity of the deflection system of the camera tube, is a large problem. Often, there are further difficulties, such as shading due to a difference in degrees of modulation, at the peripheral region and the central region in the target surface of the camera tube.
The present applicant has previously developed a novel "Color television signal generating apparatus" which overcomes the above described disadvantages by U.S. Pat. No. 4,041,528. This previously developed color television signal generating apparatus comprises a color-resolving striped filter, in a camera tube, for separating the output signal of the camera tube into required signals. Detecting means detects the envelopes of specific positive wave and negative wave signals thus separated. The output of the camera tube is a superimposed signal of a direct wave signal containing signals of three primary colors, of additional mixed colors, and a high-band component signal comprising a group of modulated color signals. This camera output signal results from the amplitude modulation of a carrier wave responsive to filter stripes in the color-resolving striped filter. The carrier wave components have a higher harmonic relation relative to two primary color signals. The separating means comprises first separating means for separating the direct signal from the above mentioned superimposed signal and second separating means for separating the high-band component signal. The envelope detecting means comprises a first detector for producing a demodulated output signal in accordance with an envelope resulting from a successive connection of peak values of the positive wave of the thus separated high-band component signal. A second envelope detection means produces a demodulated output signal in accordance with an envelope resulting from a successive connection of peak values of the negative wave of the thus separated high-band component signal.
The apparatus described above has the following advantageous features.
Since a filter comprising filter stripes of respectively equal space frequency are used for the color-resolving striped filter, moire does not occur. Since the system is not a phase separation system, stripes are not necessary for generating index pulses in the color-resolving striped filter, the camera tube, and other parts. Therefore, the color-resolving striped filter and the camera tube become simple and can be readily fabricated. Furthermore, since the rate of utilization of the incident light is improved, a bias light is unnecessary. By adjusting the spectral response characteristics of the filter stripes of the color-resolving striped filter and the spectral response characteristics of the camera tube, the output levels of the three primary color signals respectively become equal when there is a pick up of an all-color light (white light). It is easy to reduce the shading which is due to the modulation degree characteristic of the camera tube. The positive and the negative waves of the high-band component signal are, respectively, envelope detected to obtain two 2-color mixture signals. Thus, it is possible to provide a color television signal generating apparatus with an excellent performance, which is simple and can be produced at low cost.
However, in the case where the above mentioned color-television signal generating apparatus is applied to a color-television camera in which the frequency band necessary for signals and the frequency of the carrier wave are not well separated as in a small size color-television camera of simplified type, it is difficult to obtain a perfect envelope, whereby a mean value component is included, and a color error results. This has been a problem encountered heretofore. In the above mentioned apparatus, furthermore, the second harmonic component is used as it is in the color demodulation. In general, however, with respect to the characteristics of a camera tube, there is a difference between the degrees of modulation of the peripheral part and the central part of the target surface. In addition, the deterioration of the second harmonic wave frequency component in the peripheral part of the target surface is greater than that of the fundamental wave frequency component. Consequently, this gives rise to a problem in that a color shading effect is produced.
Still another problem is caused by the fact that the second harmonic wave frequency component affects the focus stability more than the fundamental wave frequency component. Since, in above described apparatus, the second harmonic wave frequency component is used for color demodulation, there has been the problem of poor focus stability. A further problem has been that the effect of noises of the second harmonic wave frequency band is not negligible and gives rise to poor S/N ratio and other difficulties.